The present invention relates to a suspension control apparatus used in vehicles.
An example of a suspension control apparatus is described in the Japanese Patent Application No. 7-135854 (1995). Such a suspension control apparatus comprises a proportional solenoid valve including a solenoid and a movable member capable of shifting in proportion to current applied to the solenoid and adapted to adjust an amount of hydraulic fluid or oil (passed through the valve) in response to the displacement of the movable member, and a suspension body disposed between a body or a chassis of a vehicle and a vehicle wheel for expansion and compression and which is capable of being expanded or compressed in response to the operation of the solenoid valve. In this apparatus, the current applied to the solenoid is adjusted by using a PWM (pulse width modulated) signal, so that a plunger of the proportional solenoid valve is positioned to provide a desired damping force. In this case, by changing a duty ratio of the PWM signal, a dither component is included in the current applied to the solenoid to afford minute vibration (dither) to the plunger, thereby facilitating the displacement of the plunger to ensure good response of the suspension control.
In the conventional technique described in the above Japanese Patent Application No. 7-135854, an amount of the current applied to the solenoid is determined by assuming that a resistance value of the solenoid of the proportional solenoid valve is constant regardless of change in temperature. Thus, due to the change in temperature during use or a difference between a set temperature and a temperature in use, the resistance value of the solenoid of the proportional solenoid valve changes to cause a difference between a desired current value (target value) to be applied to the solenoid and a current value (actual current value) actually flowing through the solenoid, with the result that there is a danger of deviation of a damping force obtained (and, accordingly, a vehicle height value) from a desired value. Such a disadvantage may also be caused by fluctuation in voltage of a battery.